1. Field of the Invention
The present invention relates to a surface-oxide abrasion-resistant lubricant coating and a method for forming the same. In particular, the present invention relates to a surface-oxide abrasion-resistant lubricant coating capable of not only enhancing properties, such as the abrasion resistance and the lubricity of a metal part (hereinafter, referred to as a “sliding contact part”, such as a mechanical part, a mold, and a cutting tool, that is used in slidable contact with an object to be contacted serving as a counterpart to be slidingly contacted, but also reducing the occurrence of wear, damage, and so forth of the object to be contacted by reinforcing a contact portion (hereinafter, referred to as a “sliding contact portion”) of the sliding contact part and improving the lubricity of the sliding contact portion and to a method for forming such a surface-oxide abrasion-resistant lubricant coating.
2. Description of the Related Art
Fluid lubricants, such as oil and grease are typically used for the lubrication of sliding contact portions. However, it may not be possible to use such fluid lubricants for design reasons or due to restrictions imposed by the operating environments, such as vacuum conditions, in which fluids or absorbed gases easily evaporate or desorb. Furthermore, with the recent growing consciousness of environmental issues, minimized use of fluid lubricants is desired because leakage of such fluid lubricants out of machines may lead to environmental disruption.
In response to these demands, solid lubricants are being increasingly used for lubrication instead of fluid lubricants. Examples of such solid lubricants include layered structures such as graphite (C), molybdenum disulfide (MoS2), tungsten disulfide (WS2), and boron nitride (BN).
In fact, in order to enhance the lubricity at a sliding contact portion by forming a coating made of such a solid lubricant over the surface of the sliding contact portion, the present inventor has proposed a method for forming an abrasion-resistant coating by ejecting powders of a solid lubricant, such as zinc, molybdenum disulfide, or tin, onto the surface of the object to be processed at a predetermined ejection pressure and ejection speed to diffuse and penetrate elements in the composition of the solid lubricant over the surface of the sliding contact portion (Japanese Patent No. 3357586).
For the formation of a solid-lubricant coating by ejecting such powders, the present inventor has also proposed a technique for ejecting a mixture of metal particles, such as tin, making up a base phase of the coating to be formed and particles of a solid lubricant such as molybdenum disulfide to form a coating having the solid lubricant dispersed in the base phase (Japanese Patent No. 3357661).
Problems with Known Layered-Structure Solid Lubricants
Limitation of Effects
Of the above-described solid lubricants, layered-structure solid lubricants such as graphite, molybdenum disulfide, tungsten disulfide and boron nitride exhibit their lubricity as a result of being decomposed into layers due to frictional contact with a sliding contact portion. However, such solid lubricants themselves do not have fluidity, unlike fluid lubricants such as oil or grease. For this reason, once decomposed, solid lubricants cannot restore their original states. This means that solid lubricants lose their lubricity once their decomposition is completed.
To overcome this problem, a system for additionally supplying a solid lubricant, when necessary, to an interface contacted with an object to be contacted serving as a counterpart to be contacted is necessary in order to allow such layered-structure solid lubricants to keep exhibiting lubricity for an extended period of time.
In regard to this point, for the invention disclosed in the Japanese Patent No. 3357661, the coating formed over the surface of a sliding contact portion is constructed such that a solid lubricant such as molybdenum disulfide is dispersed in soft metal such as tin that serves as a base phase. With this structure, molybdenum disulfide that is unbroken and that has been dispersed in the base phase such as tin emerges over the interface contacted with the object to be contacted as a result of the base phase being worn away, thereby restoring the lubricity of the molybdenum disulfide.
However, regardless of adoption of such a structure, the lubricity of a layered-structure solid lubricant such as molybdenum disulfide is restricted by the total amount of the layered-structure solid lubricant dispersed in the coating.
High Cost or Difficulty in Handling
The above-described layered-structure solid lubricants are generally expensive, except for graphite. In recent years particularly, a rapid increase in the number of cars manufactured in developing countries has stimulated the demand for molybdenum disulfide, and molybdenum disulfide is becoming not only more expensive but also more difficult to obtain.
For this reason, if such expensive molybdenum disulfide, tungsten disulfide, or boron nitride is used as a solid lubricant, the product price itself will surge, resulting in disadvantageous in terms of price competitiveness in the market.
On the other hand, among the above-described layered-structure solid lubricants, graphite is advantageous in terms of price over the other layered-structure solid lubricants. However, fine particles of graphite are difficult to handle because they are prone to dust fires or dust explosions. In particular, if graphite powder is ejected together with a compressed gas using a blasting machine as described in Japanese Patent No. 3357661, the blasting needs to be carried out under controlled conditions to prevent such a dust fire from occurring, and the use of graphite is limited for this reason.
Problems with Soft-Metal Coatings
Restrictions Imposed by Base Material
Ways of enhancing lubricity without using layered-structure solid lubricants as described above may include forming a coating of soft metal, such as tin, over the surface of a sliding contact portion.
Referring to FIGS. 4A to 4C for explaining the principle behind enhanced lubricity of the sliding contact portion by forming a coating of soft metal, the frictional force can be given by the product of the area A and the shearing strength s (A×s) of a portion condensed and solidified. In the example of FIG. 4A where hard metal is rubbed against soft metal, the shearing strength s decreases mainly because the soft metal is easily subjected to plastic deformation. However, the total frictional force represented by A×s does not decrease because the area A of the portion condensed and solidified increases due to the deformation of the soft metal.
Similarly, in the example of FIG. 4B where hard metal is rubbed against hard metal, even though the area A of the portion condensed and solidified is small because the hard metal is subjected to only minor plastic deformation, the frictional force represented by A×s does not decrease because the shearing strength s is high.
In contrast, in the example shown in FIG. 4C where a coating of soft metal is formed over hard metal, the area A of the portion condensed and solidified is small because the weight is supported by the underlying hard metal. Furthermore, because the shearing strength s is determined based on the soft metal formed over the surface, the product of A and s, that is, the friction resistance decreases.
According to the principle behind the decreased friction resistance by forming a coating of such soft metal, the lubricity achieved by forming a coating of soft metal is exhibited when the coating of soft metal is formed over relatively hard base material which has the property that no plastic deformation occurs at the time of contact with an object to be contacted. In other words, if the hardness of the base material is so low that the base material itself is subjected to plastic deformation at the time of contact with an object to be contacted, the coating of soft metal formed over the surface will exhibit only limited enhancement of lubricity.
Loss of Lubricity Due to Wear of a Coating
Enhancement of lubricity achieved by forming a coating of soft metal is seen in the form of continuous lubricity that is exhibited when soft metal with low shearing strength formed as a coating over the surface of base material undergoes repeated movement and transfer due to plastic deformation and restores the original surface. However, while repeating the above-described movement and transfer, such soft metal becomes unable to restore the original surface and is finally ejected from between the interfaces to be contacted in the form of abrasion powder. In this manner, the coating of soft metal is gradually worn away, or such abrasion powder gradually increases in amount, thus eventually losing its lubricity.
Such abrasion powder is generated probably as a result of transferred particles hardening through interaction with oxygen in the air at the friction surface.
More specifically, while being repeatedly moved and transferred in the form of transferred particles at the time of friction, the soft metal formed as a coating absorbs or chemically combines with oxygen in the air, and these transferred particles harden so that they lose plastic deformability and become unable to restore the original surface. Furthermore, the transferred particles hardening in this manner scrape the surface of the coating of soft metal or, in some cases, the object to be contacted serving as a counterpart to be contacted, to grow like a rolling snowball to such a degree that they cannot remain between the interfaces to be contacted and are ejected from between the interfaces to be contacted.
Such abrasion powders generated based on the mechanism described above cause the coating of soft metal to be gradually worn out and lose its lubricity, and moreover, transferred powder hardening as a result of oxidation damages the base material or the object to be contacted serving as a counterpart to be contacted.
In light of the shortcomings of lubrication inherent to the formation of a soft-metal coating, the inventor of the present invention hypothesized that high lubricity of a coating can be maintained for an extended period of time while still preventing the base material and the object to be contacted serving as a counterpart to be contacted from being damaged by forming a coating that exhibits high-hardness at the base material and exhibits low friction resistance and low shear resistance at the interface contacted with the object to be contacted, as well as by preventing the hardening of transferred particles generated at the time of sliding contact.
Coatings that not only exhibit high hardness at the base material and low hardness at the interface contacted with the object to be contacted but also prevent transferred particles from hardening, as described above, may be realized by the following procedure. The surface of the sliding contact portion is reinforced in advance by forming a hard layer over the surface of the sliding contact portion through carburization or nitriding or by forming a ceramic coating through CVD, PVD or the like, and this reinforced surface of the sliding contact portion is then plated with precious metal such as gold (Au) or silver (Ag) which is a relatively soft and stable substance not oxidized in the air.
However, if a coating is to be formed via this method, not only is a large, costly processing apparatus for carburization, nitriding, CVD, or PVD needed, but also a plurality of different processes including surface reinforcement and plating of precious metal must be combined to form the coating.
In addition, precious metal such as gold or silver that is the material of the coating formed over the interface contacted with the object to be contacted is expensive, and the price of the product itself having such a coating formed thereon rises accordingly, thus jeopardizing the price competitiveness in the market.
In view of these circumstances, the present invention is intended to provide a surface-oxide abrasion-resistant lubricant coating that can not only achieve high lubricity maintainable for an extended period of time but also prevent a base material and a coating from being worn out and an object to be contacted serving as a counterpart to be contacted from being damaged, via a simpler method and with less expensive material. The present invention is also intended to provide a method for forming such a surface-oxide abrasion-resistant lubricant coating without having to use a large apparatus, as well as via a simpler method.